FIGS. 1A to 1E illustrate a series of cross views of a conventional process for fabricating metal bumps. First, a substrate 100 with a plurality of pads 102 shown in FIG. 1A is provided. A patterned solder mask 110 is then formed on the substrate 100. The solder mask 110 is first formed to cover the substrate 100, and subsequently a patterning process, such as a photolithography process, is conducted on the solder mask 110 to form a plurality of first openings 112 exposing these pads 102. Next, a conductive layer 120 is formed over the patterned solder mask 110, wherein the side walls of each first opening 112 and these pads 102 are blanketed by the conductive layer 120. Subsequently, a patterned photoresist 130 shown in FIG. 1B is formed on the conductive layer 120, wherein the patterned photoresist 130 has a plurality of second openings 132 used to expose a portion of the conductive layer 120. The formation of the patterned photoresist 130 comprises forming a photoresist layer on the conductive layer 120, and conducting a patterning process, such as photolithography process, to form these second openings 132, wherein each of the second openings 132 communicates with one of the first openings 112 to expose the conductive layer 120. Refer to FIG. 1C. In FIG. 1C a metal layer 140 is formed on the patterned photoresist 130 to fully fill these first openings 112 and to partially fill each of the second openings 132. After the pattern photoresist 130 is removed, the remaining conductive layer 120 and the metal layer 140 are subsequently etched until the portion of the conductive layer 120 that is not covered by the metal layer 140 is removed. Since the conductive layer 120 is much thinner than the metal layer 140, the metal layer 140 and the portion of the conductive layer 120 beneath the metal layer 140 remains (shown in FIG. 1D) when the etching process is completed.
Generally if the metal layer 140 is made of copper, a re-flow process is not necessary, and the remaining metal layer 140 and the portion of the conductive layer 120 beneath thereof can serve as a metal bump. However, if the metal layer 140 is made of tin, a re-flow process may be required to make the remaining metal layer 140 and the portion of the conductive layer 120 beneath thereof to form a pre-solder 150 (Shown in FIG. 1E).
Since the thickness of the metal layer 140 formed via a conventional method is not uniform, the metal bumps or pre-solders each of which is constructed by the combination of the remaining metal layer 140 and the portion of the conductive layer 120 beneath thereof may vary in size. The metal bumps or pre-solders with varying size could reduce the yield, and cannot satisfy the quality requirements of the fine pitch package process, which may increase the manufacturing cost.